This invention relates to a programmable controller and deals more particularly with such a controller which includes terminals permitting a hardwired logic circuit to be connected with the controller to provide a safety override or redundancy to a portion of the control system or to otherwise augment the functions of the controller.
Programmable controllers of the type with which this invention is concerned have a wide variety of applications in the machine and process control field. They may, for example, be used to control the operation of complex machine tools, the operation of furnaces, generators and other equipment in a power station, the operation of a chemical processing facility or the operation of a material handling and/or assembly line. Through the use of a programmable memory, the controller may be readily supplied with a set of instructions to adapt it to a particular control situation, thereby allowing one design of controller to be easily and inexpensively custom tailored to a selected particular process and enabling the control function of the controller to be readily changed as changes or adjustments are made in its related process.
Programmable controllers are usually quite complex and because of this are subject to some failures or errors. The incidence of such failures and errors is generally very low, but as to some which might occur, particularly those having the potential for causing a disaster, it may be desirable to provide some control redundancy. Also, in the case of some processes or machines, a controller may be used only while the process or machine is progressing in a normal operating condition. At other times, as when the process or machine is being brought up to or shut down from its normal state, manual control may be used in place of the controller, but even during manual control, it may be desirable to have an automatic safety override to supplement the manual control in one or more critical areas.
Programmable controllers of the type in question operate by reading a plurality of input signals originating from sensors detecting various parameters of the controlled system and process such input data to provide a plurality of output signals in turn controlling the operation of valves, switches, solenoids, relays or other control devices controlling various variables of the system. Due to the time required for processing the data and the fact that the processor generally works in scans during which the different inputs are processed in sequence, some discrete amount of time usually elapses between the change in an input signal and a corresponding change in a related output signal. In some very time sensitive systems, for example, some chemical mixing processes where quantities of liquids are measured by turning on and off valves, this processor response time may be too coarse for adequate or satisfactory results.
The general object of this invention is, therefore, to provide a programmable controller to which supplementary hardwired logic circuits may be readily added to provide supplementary redundant or override control of selected parts of the controlled system for safety purposes, or to improve the response time or other characteristics of the programmable controller in certain circumstances.
A further object of the invention is to provide a means enabling ready connection of a supplementary hardwired logic circuit to a programmable controller and whereby a hardwired logic circuit connected through the use of such means uses the same input sensors, input signal converters, output signal converters, and control devices as does the programmable controller thereby eliminating the need for additional sensors, input converters, output converters or control devices.
Other objects and advantages of the invention will become apparent during the course of the following description and from the claims forming a part hereof.